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| United States Patent |
5,089,068
|
|
Okada
, et al.
|
February 18, 1992
|
Cold rolled steel sheets having improved spot weldability
Abstract
An extra-low carbon cold rolled steel sheet useful for use in automobiles
and the like, having an improved spot weldability is obtained by combining
and adding Ti, Nb and B in predetermined amounts to an extra-low carbon
steel to control the amount of fine Ti precipitate dispersed in steel to
within a specified reasonable range. Furthermore, the spot weldability is
improved by controlling the production conditions of the extra-low carbon
steel sheet to within specified reasonable ranges.
| Inventors:
|
Okada; Susumu (Chiba, JP);
Imanaka; Makoto (Chiba, JP);
Masui; Susumu (Chiba, JP);
Obara; Takashi (Chiba, JP);
Shinozaki; Masatoshi (Chiba, JP);
Tsunoyama; Kozo (Chiba, JP)
|
| Assignee:
|
Kawasaki Steel Corporation (JP)
|
| Appl. No.:
|
410414 |
| Filed:
|
September 21, 1989 |
Foreign Application Priority Data
| Jun 18, 1987[JP] | 62-150313 |
| Jun 19, 1987[JP] | 62-152977 |
| Jun 19, 1987[JP] | 62-152978 |
| Jun 19, 1987[JP] | 62-152979 |
| Current U.S. Class: |
148/328; 148/330 |
| Intern'l Class: |
C22C 038/06; C22C 038/14; C22C 038/12 |
| Field of Search: |
148/330,12 C,320
|
References Cited
U.S. Patent Documents
| 4141761 | Feb., 1979 | Abraham et al. | 148/327.
|
| 4504326 | Mar., 1985 | Tokunaga et al. | 148/12.
|
| 4586966 | May., 1986 | Okamoto et al. | 148/12.
|
| 4615749 | Oct., 1986 | Satoh et al. | 148/330.
|
| Foreign Patent Documents |
| 080809 | Jun., 1983 | EP.
| |
| 228756 | Jul., 1987 | EP.
| |
| 59-74232 | Apr., 1984 | JP.
| |
| 59-190332 | Oct., 1984 | JP.
| |
| 59-193221 | Nov., 1984 | JP.
| |
| 60-159153 | Aug., 1985 | JP | 148/330.
|
| 60-47328 | Oct., 1985 | JP.
| |
| 61-110757 | May., 1986 | JP.
| |
| 61-113724 | May., 1986 | JP.
| |
| 61-133323 | Jun., 1986 | JP.
| |
Primary Examiner: Yee; Deborah
Attorney, Agent or Firm: Parkhurst, Wendel & Rossi
Parent Case Text
This is a division of application Ser. No. 07/204,619, filed Jun. 9, 1988,
now U.S. Pat. No.4,889,566.
Claims
What is claimed is:
1. A cold rolled steel sheet comprising:
not more than 0.004 wt % C, not more than 0.1 wt % Si, not more than 0.5 wt
% Mn, not more than 0.025 wt % P, not more than 0.025 wt % S, not more
than 0.0040 wt % N, 0.01.about.0.04 wt % Ti, 0.003.about.0.008 wt % Nb,
0.0001.multidot.0.001 wt % B, 0.01.about.0.1 wt % Al and the remainder
being substantially Fe; and
fine precipitates of Ti having a grain size of not more than 0.05 .mu.m
uniformly dispersed in said steel in an amount of not less than 30 ppm as
a Ti conversion amount.
2. A cold rolled steel sheet comprising:
not more than 0.004 wt % C, not more than 0.1 wt % Si, not more than 0.5 wt
% Mn, not more than 0.025 wt % P, not more than 0.025 wt % S,
0.01.about.0.04 wt % Ti, 0.001.about.0.008 wt % Nb, 0.0001.about.0.001 wt
% B, 0.01.about.0.1 wt % Al and the remainder being substantially Fe;
wherein said steel sheet has a surface roughness satisfying either one of
the following (a) and (b);
(a) surface roughness (SRa) and yield stress (Y.S.) satisfy the following
relationship:
SRa.gtoreq.(32.4/Y.S.)-1.1;
(b) an area ratio of convex portions on a surface of said steel sheet (SSr)
of not more than 60% and an average area per one convex portion (SGr) of
not less than 2.times.10.sup.4 .mu.m.sup.2.
3. The cold rolled steel sheet of claim 1, wherein the amount of Nb present
is less than 1/5 (93/48) of the amount of Ti present.
4. The cold rolled steel sheet of claim 2, wherein the amount of Nb present
is less than 1/5 (93/48) of the amount of Ti present.
5. The cold rolled steel sheet of claim 1, wherein the amount of Nb present
is less than 1/4 of the amount of Ti present.
6. The cold rolled steel sheet of claim 2, wherein the amount of Nb present
is less than 1/4 of the amount of Ti present.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a cold rolled steel sheet useful for automobiles
and a method of producing the same, and more particularly to an extra-low
carbon cold rolled steel sheet having an improved spot weldability without
damaging excellent formability.
2. Related Art Statement
Cold rolled steel sheets having improved formabilities, particularly deep
drawability are mainly applied to inner and outer panels for automobiles.
Therefore, there have hitherto been made studies so as to provide optimum
conditions satisfying mechanical properties required for steel sheets when
the sheet is press-formed into parts for automobiles. Particularly, the
steel sheets for automobiles should be adapted to a great variety of
designs, so that the improvement of r-value corresponding to the deep
drawability or the reduction of yield stress and improvement of work
hardenability from a viewpoint of shape freezability are also important.
For this end, extra-low carbon steels having a carbon content reduced to a
level of about 10 ppm becomes adopted lately.
The production technique of extra-low carbon steel sheets for deep drawing
developed from the above viewpoints is disclosed, for example, in Japanese
Patent laid open No. 59-193,221 or in the specification of Japanese Patent
Application No. 61-219,803 previously filed by the inventors.
In these techniques, however, spot weldability and properties of weld
portion are not considered as important properties in addition to the
formability. Particularly, the extra-low carbon steel is generally poor in
spot weldability as compared with low carbon steels.
The spot welding operation is an indispensable factor in the assembling
work of parts formed by pressing or other process. Therefore, the
operability of such a spot welding as well as mechanical properties of
weld portion are important together with the formability in view of the
evaluation on total properties of the steel sheet.
Moreover, the spot weldability is barely reported in Japanese Patent laid
open No. 61-110,757, but the control of the thickness of surface oxide
film disclosed therein is very difficult to be applied to industrial
production and is hardly practical.
In general, it has been well-known to improve elongation (El) and the
Lankford value (r-value) and reduce Y.S. (low Y.R.) from a viewpoint of
formabilities, particularly deep drawability and shape freezability in the
press forming, which have been realized by extremely reducing the carbon
content of steel sheet. However, when such an extra-low carbon steel is
subjected to spot welding, the strengths are poor as compared with those
of the conventional low carbon steel and the proper welding condition is
shifted toward a high welding current side as compared with the
conventional low carbon steel as shown in FIG. 20. The result is that a
new problem arises in that the consumption of a spot welder becomes
faster.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to advantageously solve the
aforementioned problems and provide a cold rolled steel sheet having
improved spot weldability and mechanical properties of weld portion
without damaging press formability, and a method of producing the same.
At first, the invention will be described with respect to investigational
details resulting in the success of the invention.
In order to improve the formabilities or r-value and El, it is effective to
reduce the C amount, and hence the steel is softened. However, the
inventors have found out in the course of development and study on a
technique for improving the spot weldability that the local deformation is
easily caused in the excessively softened steel sheet by pressure applied
from the electrode in the spot welding and hence the contact resistance
between electrode and steel sheet or between steel sheets is abnormally
lowered (FIG. 21). Therefore, it is considered that the shifting of the
proper welding condition range in the spot welding of the extra-low carbon
steel sheet results from such a lowering of the electric resistance. Thus,
it becomes important to adjust the Y.S. (yield stress) as an adequate
property of the steel sheet.
In order to raise Y.S., it is generally performed to increase the amount of
alloying elements such as C, N, Mn, P, Si and the like, which can not
avoid the reduction of r-value and El on the other hand. Furthermore,
increasing the Y.S. can easily be achieved by reducing the skin pass
rolling or the like, but in this case the decrease of r-value and El can
not also be avoided in the conventionally known techniques. Therefore, in
order to restrain the degradation of such properties to a minimum, it is
necessary to obtain the effect of raising Y.S. by adding a smaller amount
of the alloying element. The inventors have aimed at this point and made
investigations with respect to the development of steel sheets having good
work hardening properties.
On the other hand, since the extra-low carbon steel sheet is less in the
amount of impurities and very large in the grain growth at the heating,
the coarsening of crystal grains in the weld portion and hence the
softening of the steel sheet are similarly considered to be a factor of
obstructing the weldability.
The inventors have made further studies in order to solve the
aforementioned problems and obtained knowledge that the simultaneous
addition of Ti, Nb and B to the extra-low carbon steel is very effective
for improving the strength of the spot weld portion.
And also, the inventors have made further investigations with respect to
the improvement of reasonable welding current or the like based on the
above knowledge and obtained the following discoveries:
1. It has been discovered that the work hardening properties are improved
by reducing the grain size of Ti precipitates formed through the addition
of Ti without adding a great amount of the alloying element and also Y.S.
can effectively be raised by a small reduction of skin pass rolling.
Furthermore, it has been proved that it is important how much the
precipitates having a grain size of not more than 0.05 .mu.m are dispersed
into steel for linking the dislocation introduced by skin pass rolling
with the interaction of fine precipitate to effectively raise the Y.P.
(yield point).
Moreover, it has been elucidated that in the thus obtained steel sheets,
not only the spot weldability is improved but also the properties of weld
portion such as strength and toughness are improved so as to match with
those of the base metal.
2. It has been confirmed that the strengths of spot weld portion are very
effectively improved by simultaneously adding Ti, Nb and B to the
extra-low carbon steel and controlling the coexistent state of these
elements to a particular range and subjecting the steel to adequate steps.
That is the effect of raising the hardness of the weld portion is first
obtained when the three elements of Ti, Nb and B are coexistent together,
and if either one of these elements is lacking, the improving effect of
the weldability is not observed. Even in the conventional steel sheets for
deep drawing, there are proposed many steels of Ti-Nb series, Ti-B series,
Nb-B series and the like, but these proposals aim at the effect of
improving mechanical properties by each of these elements alone (for
example, improvement of El, r-value and the like), so that the addition
effect of each element of Ti, Nb and B is independent with each other and
merely additional, which can be well be explained from the conventionally
known theory of recrystallization structure formation.
However, in order to achieve the effect aimed at by the invention, it is
necessary to add reasonable amounts of three elements Ti, Nb and B or to
coexist three elements under a delicate balance, which is largely
different from the conventional Ti, Nb or B containing steel sheet for
deep drawing. That is, all of the conventionally proposed Ti, Nb or B
containing steels for deep drawing aim at only the improvement of deep
drawability, so that they are said to be a steel containing an excessive
amount of Ti, Nb, B or the like or a steel wherein a balance among these
three elements is unsuitable in view of the spot weldability. Therefore,
it is considered that these conventional steels do not provide the good
spot weldability aimed at by the invention.
It has also found out that the production steps, particularly the skin pass
rolling step play an important role for sufficiently developing the above
effect.
In general, the skin pass rolling is not necessarily required because the
extra-low carbon steel is very small in the amount of solute element and
does not generate the yield elongation. That is, the purpose of the skin
pass rolling in the extra-low carbon steel is different from that of the
low carbon steel and is shape remedy and surface adjustment for the most,
so that it is considered that in case of the extra-low carbon steel, the
skin pass rolling is not completely needed or is sufficient at a very
slight reduction.
According to the invention, it has been succeeded that the fatigue
properties of the spot weld portion, particularly fatigue properties at a
low cycle are advantageously improved by setting the reduction of the skin
pass rolling at a value higher than the usual value without substantially
scarifying the other properties.
3. It has been discovered that the spot weldability is advantageously
improved without raising the Y.S. by performing the skin pass rolling with
a work roll subjected to dulling, preferably laser dulling to make the
surface roughness of the steel sheet large or restrict the area of convex
portion on the steel sheet surface.
That is, it has been confirmed that when the surface roughness of the steel
sheet is made large to reduce the contacting area between the steel
sheets, the electric resistance in the welding becomes large and
consequently the welding current value can be decreased.
Of course, it is possible to increase Y.S. without damaging the r-value and
El by the above items 1 and 2, but the invention is suitable for
applications not requiring the increase of Y.S. such as an application
that the shape freezability or difference in size between working press
mold and steel sheet after the working is required to be very small.
The invention is based on the aforementioned knowledges.
According to a first aspect of the invention, there is the provision of a
cold rolled steel sheet having improved strength and toughness in weld
portion, characterized in that said steel comprises not more than 0.004 wt
% of C, not more than 0.1 wt % of Si, not more than 0.5 wt % of Mn, not
more than 0.025 wt % of P, not more than 0.025 wt % of S, not more than
0.0040 wt % of N, 0.01.about.0.04 wt % of Ti, 0.003.about.0.010 wt % of
Nb, 0.0001.about.0.0010 wt % of B, 0.01.about.0.10 wt % of Al and the
remainder being substantially Fe, and fine precipitates of Ti having a
grain size of not more than 0.05 .mu.m are uniformly dispersed into said
steel in an amount of not less than 30 ppm as a Ti conversion amount.
According to a second aspect of the invention, there is the provision of a
cold rolled steel sheet having improved formability and spot weldability,
characterized in that said steel comprises not more than 0.004 wt % of C,
not more than 0.1 wt % of Si, not more than 0.5 wt % of Mn, not more than
0.025 wt % of P, not more than 0.025 wt % of S, 0.01.about.0.04 wt % of
Ti, 0.001.about.0.010 wt % of Nb, 0.0001.about.0.0010 wt % of B,
0.01.about.0.10 wt % of Al and the remainder being substantially Fe, and
has a surface roughness satisfying either one of the following (a) and
(b):
(a) surface roughness (SRa) and yield stress (Y.S.) satisfy the following
relationship:
SRa.gtoreq.(32.4/Y.S.)-1.1;
(b) an area ratio of convex portions on the surface of said steel sheet
(SSr) is not more than 60% and an average area per one convex portion
(SGr) is not less than 2.times.10.sup.4 .mu.m.sup.2.
According to a third aspect of the invention, there is provided a method of
producing a cold rolled steel sheet having improved strength and toughness
in weld portion, which comprises subjecting molten steel comprising not
more than 0.004 wt % of C, not more than 0.1 wt % of Si, not more than 0.5
wt % of Mn, not more than 0.025 wt % of P, not more than 0.025 wt % of S,
not more than 0.0040 wt % of N, 0.01.about.0.04 wt % of Ti,
0.003.about.0.010 wt % of Nb, 0.0001.about.0.0010 wt % of B,
0.01.about.0.10 wt % of Al and the remainder being substantially Fe to a
solidification and cooling step, during which said molten steel is cooled
at a cooling rate of not less than 3.degree. C./min within a temperature
range of at least 1,300.degree..about.1,000.degree. C., and heating the
resulting slab to a temperature of not higher than 1,200.degree. C., and
subjecting said slab to hot rolling and cold rolling, and then subjecting
the slab to a continuous annealing within a temperature range of
700.about.900.degree. C.
According to a fourth aspect of the invention, there is provided a method
of producing a cold rolled steel sheet having improved spot weldability,
which comprises hot rolling a slab of steel comprising not more than 0.004
wt % of C, not more than 0.1 wt % of Si, not more than 0.5 wt % of Mn, not
more than 0.025 wt % of P, not more than 0.025 wt % of S, not more than
0.0040 wt % of N, 0.01.about.0.04 wt % of Ti, 0.001.about.0.010 wt % of
Nb, 0.0001.about.0.0010 wt % of B, 0.01.about.0.10 wt % of Al and the
remainder being substantially Fe, and satisfying the following relations
(1).about.(4):
(11/93)Nb-0.0004.ltoreq.B.ltoreq.(11/93)Nb+0.0004 (1)
Ti>(48/12)C+(48/14)N (2)
NB<1/5.multidot.(93/48)Ti (3)
C+(12/14)N+(12/11)B>0.0038 (4)
under conditions that a finish temperature is 700.degree..about.900.degree.
C. and a coiling temperature is 300.degree..about.600.degree. C., and cold
rolling the resulting hot rolled sheet at a reduction of 60.about.85%, and
subjecting the resulting cold rolled sheet to a continuous annealing
within a temperature range of from recrystallization temperature to not
higher than 780.degree. C.
According to a fifth aspect of the invention, there is provided a method of
producing a cold rolled steel sheet having improved fatigue properties in
spot weld portion, which comprises hot rolling a slab of steel comprising
not more than 0.004 wt % of C, not more than 0.1 wt % of Si, not more than
0.5 wt % of Mn, not more than 0.025 wt % of P, not more than 0.025 wt % of
S, not more than 0.0040 wt % of N, 0.01.about.0.04 wt % of Ti,
0.001.about.0.010 wt % of Nb, 0.0001.about.0.0010 wt % of B,
0.01.about.0.10 wt % of Al and the remainder being substantially Fe, and
satisfying the following relations (1).about.(4):
(11/93)Nb-0.0004.ltoreq.B.ltoreq.(11/93)Nb+0.0004 (1)
Ti>(48/12)C+(48/14)N (2)
Nb<1/5.multidot.(93/48)Ti (3)
C+(12/14)N+(12/11)B>0.0038 (4)
under conditions that a finish temperature is 700.degree..about.900.degree.
C. and a coiling temperature is 300.degree..about.600.degree. C., and cold
rolling the resulting hot rolled sheet at a reduction of 60.about.85%, and
subjecting the resulting cold rolled sheet to a continuous annealing
within a temperature range of from recrystallization temperature to not
higher than 780.degree. C., and then subjecting to a skin pass rolling at
a reduction of not less than (sheet gauge (mm)+0.1)% but not more than
3.0%.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described with reference to the accompanying
drawings, wherein:
FIG. 1 is a graph showing an influence of Ti, Nb and B addition upon spot
weldability;
FIG. 2 is a graph showing an influence of Ti, Nb and B addition upon
hardness of weld portion;
FIG. 3 is a graph showing a relation between Y.S. of steel sheet and range
of reasonable welding current;
FIG. 4 is a graph showing a relation between amount of Ti precipitate
having a grain size of not more than 0.05 .mu.m as Ti conversion amount
and rising rate of Y.P. through skin pass rolling;
FIG. 5 is a comparison graph showing amounts of Ti precipitates having a
grain size of not more than 0.05 .mu.m or more than 0.05 .mu.m as a
parameter of Ti/N;
FIG. 6 is a graph showing a relation between Ti/N ratio and amount of Ti
precipitate having a grain size of not more than 0.05 .mu.m;
FIG. 7 is a graph showing a relation among slab cooling rate from
1,300.degree. C. to 1,000.degree. C., total amount of Ti precipitate and
amount of fine precipitate;
FIG. 8 is a graph showing a relation among slab heating temperature, total
amount of Ti precipitates and amount of fine precipitate;
FIG. 9 is a graph showing a relation between amount of Ti precipitate
having a grain size of not more than 0.05 pm as Ti conversion amount and
hammering brittle temperature;
FIG. 10 is a graph showing a relation between fracture unit of brittleness
and hammering brittle temperature;
FIG. 11 is a relation between addition amount of Nb and B and hardness of
spot weld portion;
FIG. 12 is a graph showing an influence of Nb/Ti upon El of steel sheet;
FIG. 13 is a graph showing an influence of Ti amount upon hardness of weld
portion;
FIG. 14 is a graph showing an influence of C, N and B upon hardness of weld
portion;
FIG. 15 is a graph showing a relation between reduction of skin pass
rolling and lower limit of reasonable welding current;
FIG. 16 is a graph showing influences of components in steel and reduction
of skin pass rolling upon fatigue strength of spot weld portion;
FIG. 17 is a graph showing a relation between surface roughness SRa of
steel sheet and lower limit of reasonable welding current in the spot
welding;
FIG. 18 is a graph showing influences of surface roughness SRa and yield
stress Y.S. of steel sheet upon lower limit of reasonable welding current;
FIG. 19 is a graph showing a relation between area ratio of convex portions
SSr and average area per one convex portion SGr exerting on cross tensile
strength after spot welding;
FIG. 20 is a graph showing reasonable welding conditions in the
conventional low carbon steel and the extra-low carbon steel; and
FIG. 21 is a graph showing a relation between Y.S. and electric resistance
in steel sheet.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention will be described in detail below.
FIG. 1 shows results obtained by examining an influence of addition of Ti,
Nb and B, which are particularly important components in the invention,
upon the spot weldability.
As a test steel for this experiment, there were used the usually used low
carbon steel for deep drawing containing C: 0.04%, Si: 0.01%, Mn: 0.20%,
P: 0.01%, N: 0.0040% and Al: 0.036%, the conventional Ti added extra-low
carbon steel based on C: 0.002%, Si: 0.1%, Mn: 0.1%, P: 0.01%, S: 0.01%,
Al: 0.02% and N: 0.002.about.0.003% and containing Ti: 0.06%, and Ti-Nb-B
added extra-low carbon steel according to the invention containing Ti:
0.03%, Nb: 0.005% and B: 0.0007%.
Moreover, the spot welding was carried out by welding a specimen of
0.8.times.30.times.30 mm under an applied pressure of 190 kgf through CF
type electrode of 4.5 mm in diameter with reference to a value recommended
by RWMA (Resistance Welder Manufacturer's Association).
In this case, the lower limit of the reasonable welding current is a point
that a nugget zone formed by the welding is not less than 3.sqroot.t mm (t
is sheet gauge of specimen, mm), while the upper limit thereof is a point
of generating expulsion.
As seen from FIG. 1, in the conventional Ti added extra-low carbon steel,
the reasonable welding current considerably shifts toward a high current
side as compared with the case of the conventional low carbon steel,
resulting in the requirement of large welding equipment, while in the
Ti-Nb-B added extra-low carbon steel according to the invention, the lower
limit of the reasonable welding current is approximately equal to that of
the low carbon steel, while the upper limit of the reasonable welding
current regulated by the occurrence of expulsion is shifted toward a high
current side as compared with that of the low carbon steel, so that the
range of the reasonable welding current is more enlarged as compared with
that of the low carbon steel.
Such an effect is considered to result from the optimization of softening
degree of the steel sheet. FIG. 3 shows results obtained by examining a
relation between Y.S. of the steel sheet and range of the welding current.
A slab of steel obtained by varying the C amount within a range of 0.002%
to 0.4% (Si: 0.01%, Mn: 0.1.about.0.3%, P: 0.01.about.0.02%, S:
0.01.about.0.02%, N: 0.002.about.0.005%, Al: 0.01.about.0.04%, Ti: 0.03%,
Nb: 0.005%, B: 0.0007%) was heated to 1,100.degree..about.1,250.degree. C.
and hot rolled at a finish temperature of 700.degree..about.1,000.degree.
C. and a coiling temperature of 450.degree..about.700.degree. C., and the
resulting hot rolled sheet was cold rolled at a reduction of 60.about.85%,
and thereafter the resulting cold rolled sheet was subjected to a
continuous annealing within a temperature range of
700.degree..about.880.degree. C. to obtain sheets having various Y.S.
values.
The spot welding was carried out in the same manner as in the case of FIG.
1 except that the thickness of the specimen was 0.7 mm, the welding time
was 7 cycles and the applied pressure was 175 kgf.
As seen from FIG. 3, the reasonable welding current range is strongly
affected by the Y.S. value of the steel sheet. When Y.S. is lower than 19
kgf/mm.sup.2, the reasonable welding current range considerably shifts
toward a high current side.
In order to harden the steel sheet while maintaining the good deep
drawability, it is effective to simultaneously add Ti, Nb and B to the
extra-low carbon steel.
In the following Table 1 are shown results measured on the mechanical
properties of low carbon steel and extra-low carbon steel having various
chemical compositions. In this case, the composition and production
conditions of the test steel are the same as in FIGS. 1 and 3 except that
Ti, Nb and B were properly added within ranges of Ti: 0.02.about.0.04%,
Nb: 0.005.about.0.008% and B: 0.0005.about.0.0008%, respectively.
TABLE 1
______________________________________
Y.S. T.S. El
Kind of steel (kgf/mm.sup.2)
(kgf/mm.sup.2)
(%) r-value
______________________________________
Extra-low
addition of
20.8 31.2 48.5 2.0
carbon Ti, Nb, B
steel addition of
14.1 30.4 48.1 2.0
sheet Nb, B
addition of
11.6 30.2 48.0 1.8
Ti, B
addition of
12.2 29.8 48.3 1.9
Ti, Nb
no addition
10.3 28.7 48.8 1.5
Low carbon steel sheet
22.0 33.0 45.2 1.7
______________________________________
As seen from Table 1, in the steel sheet added with three elements of Ti,
Nb and B, Y.S. is considerably improved as compared with the other
extra-low carbon steel sheets, while El and r-value are substantially
unchanged, and there is no degradation of the deep drawability. In this
connection, the low carbon steel has Y.S. level approximately equal to
that of the Ti-Nb-B added extra-low carbon steel, but the deep drawability
is remarkably poor as compared with that of the extra-low carbon steel.
In FIG. 2 are shown results measured on the hardness of weld portion when
the steel sheets shown in Table 1 are subjected to a spot welding.
As seen from FIG. 2, the base metal hardness approximately equal to that of
the low carbon steel is obtained in the Ti Nb-B added extra-low carbon
steel according to the invention, while in the other extra-low carbon
steels lacking either one of Ti, Nb and B, only the low base metal
hardness is obtained.
Furthermore, the Ti-Nb B added extra-low carbon steel according to the
invention has an advantage that the hardness of the nugget zone is high as
compared with the other extra-low carbon steels. In general, when the
hardness of the spot weld portion or its neighborhood is low, the spot
weld portion is undesirably fractured before the fracture of the base
metal and the welding strength can not sufficiently be raised. In this
connection, the hardness of weld portion in the conventional extra-low
carbon steel is insufficient.
When the hardness of weld portion is raised to such an extent that the base
metal is fractured before the fracture of the spot weld portion, even if
the hardness is increased thereabove, the spot welding strength is not
affected principally. The steel according to the invention and the
conventional low carbon steel correspond to such a state.
Of course, such an effect is not obtained only by merely adding Ti, Nb and
B to the steel sheet. That is, the reasonable addition range of each
element of Ti, Nb and B based on some metallurgical interactions as well
as the requirements for producing desired texture are existent for
obtaining this effect.
FIG. 4 shows results measured on the influence of Ti fine precipitate upon
Y.P. of steel as a relation between the amount of Ti precipitate having a
grain size of not more than 0.05 .mu.m as Ti conversion amount and the
rising rate of Y.P. through skin pass rolling (reduction: 0.8%). As seen
from FIG. 4, when the amount of fine precipitate as Ti conversion amount
is not less than 30 ppm, the rising rate of Y.P. (.DELTA.Y.P.) increases
even at the same reduction of skin pass rolling.
Such a rising rate of Y.P. is advantageous to prevent the degradation of
the weldability resulted from the excessive softening in the extra-low
carbon steel. That is, when the fine Ti precipitates are dispersed into
steel, Y.S. rises at a small reduction of skin pass rolling in the
extra-low carbon steel, and consequently the contact resistance increases
in the spot welding, so that the heat generating efficiency can be
increased at the same welding current.
The feature of the invention lies in the elucidation of conditions that the
above change advantageously acts to the mechanical properties.
And also, this is advantageous to improve the properties of weld portion.
That is, such an effect is obtained even at TIG and MIG weld portions
likewise the spot weld portion. When the steel sheet is heated at a high
temperature for a short time in the welding, the Ti precipitates finely
dispersed in steel not only restrains the coarsening of the structure in
the heating but also serves as a steel transformation site in the cooling
to make the structure of the weld portion very fine, and consequently the
strength and toughness of the weld portion are improved.
In the steel according to the invention, the dispersion effect of Ti
precipitate is enhanced by the combined addition of Nb and B.
Moreover, the press formability and the like naturally required in the thin
steel sheet besides the above effect are sufficiently compensated by
reducing the C amount to not more than 40 ppm as far as possible.
The reason why the chemical composition of the steel according to the
invention is limited to the above range is based on the following facts.
C:
In order to improve El and r-value by softening of steel, it is
advantageous to reduce the C amount as far as possible. When the C amount
exceeds 0.0040%, the mechanical properties begin to largely degrade, so
that the upper limit is 0.0040%.
Si, Mn:
Each of Si and Mn effectively acts as a deoxidizing agent, but the
excessive addition amount causes the damage of ductility. Therefore, the
upper limit is Si: 0.1% and Mn: 0.5%, respectively.
P, S:
Since each of P and S is an impurity element, it is desirable to reduce
these elements as far as possible. Each of these elements is allowed to be
not more than about 0.025%.
N:
It is advantageous to reduce the N amount as far as possible likewise the C
amount for softening the steel sheet to improve El and r-value. When the N
amount exceeds 0.0040%, the properties such as formability, resistance to
aging and the like begin to largely degrade, so that the upper limit is
0.0040%.
The fine precipitate of Ti is mainly TiN, so that the formation of fine TiN
as Ti precipitate may more advantageously be attained by controlling the
amounts of Ti and N. That is, the Ti precipitates having a grain size of
not more than 0.05 .mu.m are obtained by limiting the weight ratio of Ti
to N to a range of 1.7.about.6.8.
When the same amount of Ti precipitate having a grain size of not more than
0.05 .mu.m is ensured at the weight ratios Ti/N of 4.0 and 8.0, the amount
of Ti precipitate having a grain size of more than 0.05 .mu.m was measured
to obtain results as shown in FIG. 5.
As seen from FIG. 5, when Ti/N (=4.0) is within a range of 1.7.about.6.8,
the fine Ti precipitates are obtained on average as a whole.
The increase in the amount of coarse Ti precipitate means that the useless
Ti precipitate exhibiting a weak dispersing effect is included in a great
amount, so that it is not only disadvantageous in the effective
utilization of the aforementioned Ti precipitate but also causes the
degradation of the formability and the rise of the cost.
When the ratio Ti/N is less than 1.7, the TiN amount becomes less to the N
amount and the sufficient amount of solute B can not be ensured, while
when it exceeds 6.8, the absolute amount of TiN increases, but the ratio
of fine precipitate reduces, so that it is desirable to add Ti and N so as
to satisfy the range of weight ratio Ti/N of 1.7.about.6.8.
FIG. 6 shows a relation between Ti/N ratio and amount of fine Ti
precipitate having a grain size of not more than 0.05 .mu.m, from which it
is apparent that the better results are obtained when the ratio Ti/N is
within a range of 1.7.about.6.8.
Moreover, when the S amount is limited to not more than 0.01% at the above
defined range of the ratio Ti/N, the precipitation of TiS is suppressed to
prevent needless disappearance of Ti, which is particularly advantageous
for more enhancing the precipitation of fine Ti precipitate.
Al:
Al is added in an amount of not less than 0.01% for providing the
deoxidizing effect. However, the upper limit of Al added is 0.10% in order
to prevent the bad influence upon the mechanical properties as an
impurity.
Nb:
Nb is an element useful for making the structure of spot weld portion to
raise the hardness of the weld portion under the coexistence with B.
Furthermore, Nb effectively contributes to raise Y.P. with holding high El
and r-value by the combined addition with Ti.
This effect is appeared when the Nb amount is 0.010%, the excessive rise of
Y.P. and the decrease of El are brought about, so that the amount is
limited to a range of 0.001.about.0.010%. Moreover, it is desirable to add
Nb in an amount of not less than 0.003% in order to finely disperse the Ti
precipitate.
However, if it is intended to raise Y.S. irrespective of Ti precipitate, as
the ratio of Nb to Ti becomes high, the amount of NbC precipitated
increases to degrade the mechanical properties, so that the coiling
temperature should be not lower than 600.degree. C. and consequently the
amount of Nb added is necessary to be reduced in balance with Ti.
Particularly, when the atomic ratio of Nb to Ti is not less than 0.2, the
degradation of mechanical properties is poor, so that the ratio of Nb to
Ti is necessary to be Nb/Ti<1/5 as an atomic ratio or Nb<1/5(93/48)Ti as a
weight ratio.
In FIG. 12 are shown results examined with respect to the influence of
Nb/Ti (atomic ratio) upon El. As seen from FIG. 12, El rapidly lowers when
Nb/Ti is not less than 0.2.
B:
B is useful for raising the strengths of spot weld portion and base metal,
particularly Y.S. by adding in a slight amount in the presence of Nb
and/or Ti. This effect is recognized by adding not less than 0.0001% of B,
but when the amount is too large, the degradation of mechanical properties
is caused, so that the upper limit is 0.0010%.
Moreover, in order to satisfactorily develop the above effect irrespective
of Ti precipitate, the B amount is insufficient to merely satisfy the
above range, and is important to be limited to a range of
(11/93)Nb-0.0004.ltoreq.B.ltoreq.(11/93)Nb+0.0004 in balance with the Nb
amount.
The coexisting effect of Nb and B was examined to obtain the following
result.
FIG. 11 shows a relation between addition amount of Nb and B, and hardness
of spot weld portion (nugget zone).
As a test steel, there was used a steel having a sheet gauge of 0.8 mm and
a chemical composition based on C: 0.0015.about.0.0040%, Mn:
0.13.about.0.033%, S: 0.008.about.0.025%, P: 0.011.about.0.018%, Al:
0.022.about.0.035%, N: 0.0011.about.0.0033% and Ti: 0.015.about.0.037% and
varying amounts of B and Nb within ranges of 0.about.0.0010% and
0.about.0.011%. The spot welding conditions were the same as in FIG. 1.
As seen from FIG. 11, the hardness of the weld portion (nugget zone) is
large at Nb: 0.001.about.0.010% and B: 0.0001.about.0.0010%, and
particularly the better result is obtained when Nb and B satisfy the above
ranges and the B amount is within a range of (11/93)Nb.+-.0.0004(%).
The above result shows that the hardness of the spot weld portion is
maximum when B and Nb are existent in approximately equal atomic numbers,
and suggests a possibility that there is some interaction between Nb and B
in steel. At the present, it can not be decided whether or not this is a
direct interaction between substitution type solute atom and intersticial
solute atom at, for example, solid solution state.
Moreover, the change of properties of base metal by the combined addition
of Ti, Nb and B is also considered to result from the above interaction
between Nb and B. That is, it is considered that the above interaction
makes the crystal grain size of the hot rolled sheet fine and the crystal
grain size of the annealed sheet relatively fine to increase Y.S. and the
same time the fine homogenization of grain size of the hot rolled sheet
brings about the improvement of r-value and El.
Ti:
Ti is not only useful for fixing solute components such as N, S, C and the
like, but also exhibits a great effect for the improvement of mechanical
properties by the formation of precipitate with these elements.
The improving effect of spot weldability through Nb and B is not realized
in the absence of Ti as previously shown in FIG. 5. Because it is required
to fix a greater part of elements such as N, C and the like in steel,
which fix Nb or B as a precipitate, with Ti for causing the sufficient
interaction between Nb and B. Therefore, if it is not particularly
intended to improve the mechanical properties through the precipitation
distribution, Ti is necessary to be added in an amount of not less than
C+N (atomic number) or Ti>(48/12.multidot.C+48/14.multidot.N).
Furthermore, when Ti is added in an amount of less than 0.01% as an
absolute amount, the fixation of the solute element is insufficient and
the addition effect of Nb and B is not satisfactorily developed.
As to the deep drawability, the high r-value and El are obtained at
Ti.gtoreq.0.01%, but the excessive addition of Ti brings about the extreme
softening based on the C fixation, which badly affects the effect of the
invention. Therefore, the upper limit is 0.04%. Moreover, the presence of
the reasonable Ti amount has an effect of restraining the occurrence of
fine precipitate containing Nb, so that the coiling temperature after the
hot rolling is not necessary to be high (>600.degree. C.) as in the usual
Nb addition, which is advantageous in economy, and the excessive softening
due to the growth of crystal grain can be prevented.
As mentioned above, Ti is added in an amount of 0.01.about.0.04%,
preferably Ti/(48/12.multidot.C+48/14.multidot.N)>1. In order to obtain
the above effect at maximum, it is more advantageous to limit the Ti
amount added to a minimum.
In FIG. 13 are shown results examined on the influence of Ti amount upon
the hardness of the weld portion over a wide composition range. The
chemical composition and welding conditions are the same as in the case of
FIG. 11.
As seen from FIG. 13, the data of the hardness are roughly divided into
three parts in accordance with the range of the Ti amount. That is, in
case of Ti.ltoreq.(48/12.multidot.C+48/14.multidot.N), the weld portion
exhibits a high hardness or a very low hardness, so that the scattering of
the hardness is large. This is considered due to the fact that the Ti
amount is less so that the yield of B lowers and the interaction effect
between Nb and B is insufficient. On the other hand, in case of
Ti>(48/12.multidot.C+48/14.multidot.N), the hardness is Hv.gtoreq.180 at
minimum. Furthermore, it has been confirmed that the high level when
Ti<(48/12.multidot.C+48/14.multidot.N+48/32.multidot.S). This shows that
when Ti is added in a necessary minimum amount or an amount of not less
than equivalent to C and N, the sufficient hardness is obtained but when
the Ti addition amount is more than equivalent to S, the hardness of the
weld portion tends to rather lower. Because, it is considered that when Ti
is existent in a sufficient (excessive) amount to C, N and S, the effect
of Nb forming a precipitate with a part of C is substantially lost.
According to the invention, therefore, the expected effect is obtained by
limiting the Ti amount to Ti>(48/12.multidot.C+48/14.multidot.N), but in
order to provide a more excellent effect, it is preferable to limit the Ti
amount to a narrower range of
Ti<(48/12.multidot.C+48/14.multidot.N+48/32.multidot.S) in balance with C,
N and S.
Moreover, in case of utilizing no fine Ti precipitate, even when Ti, Nb and
B are added, if the amounts of C, N and B are too small, the hardening of
the weld portion is insufficient. FIG. 14 shows results examined on the
influence of C, N and B as an intersticial solute element upon the
hardness of the weld portion in various steels, wherein
C+12/14.multidot.N+12/11.multidot.B is plotted on an abscissa for
converting the amount of all elements into C amount.
As seen from FIG. 14, when C+12/14.multidot.N+12/11.multidot.B is not more
than 38 ppm, the effect of forming the fine structure is insufficient and
the sufficient hardness of the weld portion is not obtained. According to
the invention, therefore, C, N and B are added so as to satisfy
C+12/14.multidot.N+12/11.multidot.B>38 ppm.
Although the necessary components according to the invention are within the
aforementioned ranges, it is very effective to disperse the fine Ti
precipitate having a grain size of not more than 0.05 .mu.m into steel in
the predetermined amount as mentioned above in order to satisfactorily
achieve the object aiming at the invention.
As mentioned above, the amount of fine Ti precipitate in steel is limited
to not less than 30 ppm as a Ti conversion amount in order to effectively
obtain the .DELTA.Y.P. raising. Furthermore, the reason why the grain size
of Ti precipitate is limited to not more than 0.05 .mu.m is due to the
fact that when the grain size exceeds 0.05 .mu.m, even if the amount of
the Ti precipitate increases, the weldability and the strength and
toughness of the weld portion can not be improved to an expected extent.
According to the invention, the advantageous effect is also produced by
controlling the surface properties of the steel sheet, which is proved
from the following experimental results.
As a test steel, there were used cold rolled sheets of a low carbon steel
sheet and an extra-low carbon steel each having a chemical composition
shown in the following Table 2.
TABLE 2
__________________________________________________________________________
C Si Mn P S Ti Nb B Al N
__________________________________________________________________________
Low carbon
0.04
0.02
0.15
0.012
0.009
0.020
0.004
0.0003
0.04
0.0020
steel
Extra-low
0.002
0.02
0.14
0.014
0.011
0.017
0.006
0.0005
0.07
0.0013
carbon
steel
__________________________________________________________________________
Each of these cold rolled steel sheets was subjected to a skin pass rolling
at a reduction of 0.8% with a skin pass roll dulled at its surface by a
laser. In this case, the surface roughness pattern of the steel sheet
after the skin pass rolling was changed by varying conditions in the laser
dulling process. Then, a specimen of 30.times.30 mm was cut out from each
of the sheets and subjected to a spot welding.
FIG. 17 shows a relation between lower limit of weldable current and
surface roughness (SRa) in the spot welding. The spot welding conditions
were a sheet gauge of 0.7 mm, a welding time of 7 cycles, an applied
pressure of 175 kgf, and a cap diameter of 4.0 mm. Moreover, the surface
roughness SRa is a center-face average roughness (unit: .mu.m) and is
represented by the following equation:
##EQU1##
when a portion of area S.sub.M is taken out from the rough curved surface
at its center face and X-axis and Y-axis of orthogonal coordinate is
placed on the center face of this portion and an axis perpendicular to the
center face is Z-axis to express the rough curved surface by Z=f(X,Y),
provided that L.sub.X .multidot.L.sub.Y =S.sub.M.
As seen from FIG. 17, the lower limit of weldable current lowers with the
increase of SRa, and when SRa=2.0 .mu.m, the lower limit of weldable
current in the extra-low carbon steel becomes approximately equal to that
of the low carbon steel.
The reason on the lowering of the lower limit of weldable current with the
increase of SRa is considered as follows. That is, as the surface
roughness becomes large, the contact area in the welding becomes small. If
the same current is applied, the smaller the contact area, the larger the
electric resistance, so that the heat generating amount increases.
Therefore, as the surface roughness becomes larger, the current value for
obtaining the same heat generating amount may be made small.
It has been found out that the spot weldability of the extra-low carbon
steel is dependent upon the surface roughness SRa as mentioned above.
Further, it has been elucidated from results of many experiments that the
spot weldability is strongly dependent upon Y.S.
In this connection, the inventors have made the experiment by changing SRa
and Y.S. over wide ranges.
In FIG. 18 are shown results measured on the limit value of weldable
current by changing SRa and Y.S. when using the extra-low carbon steel of
FIG. 17. The spot welding conditions were a specimen size of
0.8.times.30.times.30 mm, a CF type electrode of 4.5 mm in diameter, an
applied pressure of 190 kgf, a welding time of 8 cycles, and a welding
current of 7.5 kA. Moreover, numerals in FIG. 18 indicate a lower limit of
weldable current at each point, respectively.
As seen from FIG. 18, when SRa satisfies SRa.gtoreq.32.4/Y.S.-1.1, the
lower limit of weldable current approximately equal to that of the low
carbon steel is obtained.
Although the reason why the lower limit of weldable current shifts toward
low SRa side as Y.S. becomes higher is not necessarily clear, the
following is considered at the present. That is, when SRa is same, as Y.S.
becomes higher, the deformation under pressure is small and consequently
the contact area in the welding becomes small, the electric resistance
rises and the heat generating amount increases. Therefore, as Y.S. becomes
higher, the lower limit of weldable current shifts up to a low SRa side.
As mentioned above, when SRa is adjusted in such a manner that SRa and Y.S.
satisfy the above relationship, good formability and spot weldability are
obtained. According to the inventors' studies, it has been confirmed that
the object aimed at the invention is further achieved by defining area
ratio of convex portions on the steel sheet surface SSr and an average
surface ratio per one of convex portions SGr within predetermined ranges.
In FIG. 19 are shown results examined on a relation between area ratio of
convex portions (SSr) and average area per one convex portion (SGr)
exerting on cross tensile strength after the spot welding of the extra-low
carbon steel used in FIGS. 17 and 18.
As a specimen for cross tensile test, there was used a specimen of 0.8 mm
in sheet gauge according to JIS Z3137. The spot welding conditions were a
welding time of 8 cycles, an applied pressure of 175 kgf, and a welding
current of 7.5 kA. The area ratio of convex portions (SSr) and average
area per one convex portion (SGr) were measured by means of a
three-dimensional surface roughness meter. The numerical value in FIG. 19
is a shearing tensile force of spot weld portion at each point.
As seen from FIG. 19, when SSr.ltoreq.60% and SGr.gtoreq.2.times.10.sup.4
.mu.m.sup.2, the shearing tensile force is not less than 300 kgf/spot and
the strength is considerably increased.
The reason on the existence of the above reasonable range as to the
strength of weld portion is considered due to the following fact. That is,
the lower the area ratio of convex portions, the smaller the contact area,
so that the electric resistance in the welding increases to lower the
welding current value. On the other hand, however, the strength after the
welding lowers as the area ratio of convex portions becomes small.
Therefore, in order to compensate the strength after the welding, it is
considered that a minimum line is existent in the average area per one
convex portion as the area ratio of convex portions is low.
The inventors have made studies based on the above fundamental data and
found out that cold rolled steel sheets having improved formability and
spot weldability are obtained by controlling the surface state of the
sheet as mentioned later.
In a first preferable embodiment, SRa is desirable to be
SRa.gtoreq.32.4/Y.S.-1.1. If SRa<32.4/Y.S.-1.1, the spot weldability based
on the surface control is not observed.
In a second preferable embodiment, SSr and SGr are desirable to be
SSr.ltoreq.60% and SGr.gtoreq.2.times.10.sup.4 .mu.m.sup.2. If SSr>60% or
SGr<2.times.10.sup.4 .mu.m.sup.2, the improved spot weldability based on
the surface control can not be obtained.
Moreover, it is naturally possible to develop the desired effect by each of
the requirements on the steel composition, particularly balance of
Ti-Nb-B, Ti precipitate and surface roughness alone. Since the conditions
satisfying each of these requirements are not contrary to each other, the
greater effect can be obtained by combining these requirements without any
troubles. For instance, the surface roughness can be adjusted in order to
more advantageously provide the reasonable welding range of the steel
sheet containing finely distributed Ti precipitates. That is, it is
desirable to simultaneously satisfy the above requirements in order to
obtain the best spot weldability.
The reason on the limitation of production conditions according to the
invention will be described in detail below.
The cooling rate in the solidification and cooling stage of steel is
particularly important for obtaining fine Ti precipitates. That is, it is
necessary to cool the steel at a cooling rate of not less than 3.0.degree.
C./min within a temperature range of 1,300.degree. C. to 1,000.degree. C.
In FIG. 7 are shown quantitatively analyzed results on the amount of Ti
precipitate having a grain size of not more than 0.05 .mu.m and the total
amount of Ti precipitates when the cooling over a temperature range of
1,300.degree. C. to 1,000.degree. C. at the casting stage is carried out
by varying the cooling rate within a range of 0.5.degree. C./min to
5.degree. C./min.
As seen from FIG. 7, the total amount of Ti precipitates reduces with the
increase of the cooling rate, while the amount of Ti precipitate having a
grain size of not more than 0.05 .mu.m inversely increases. Particularly,
when the cooling rate is not less than 3.0.degree. C./min, the fine Ti
precipitate having a grain size of not more than 0.05 .mu.m is stably
precipitated in a great amount.
Moreover, such a cooling rate can not be attained even in the ingot making
process as well as the usual continuous casting process, so that it is
necessary to take any means such as forced cooling, control of slab
thickness or the like in order to ensure the given cooling rate. This is
not necessary in another method according to the invention as mentioned
later.
Then, the slab cooled at the above cooling rate is heated at subsequent
slab heating stage, but in this case, it is required to heat the slab at a
relatively low temperature of not higher than 1,200.degree. C. for
preventing the coarsening of Ti precipitate.
In FIG. 8 are shown results examined on a relation among slab heating
temperature, total amount of Ti precipitates and amount of fine
precipitate having a
As seen from FIG. 8, when the slab heating temperature exceeds
1,200.degree. C., the amount of fine Ti precipitate rapidly reduces due to
Ostwald's growth of Ti precipitate, so that the slab heating is carried
out at a temperature of not higher than 1,200.degree. C. in the invention.
On the other hand, when Y.S. is increased irrespective of Ti precipitate or
when it is difficult to perform the quenching of the slab, it is important
that the properties such as r-value, El and the like are not degraded. In
the Ti-Nb B addition system, therefore, the restriction of conditions at
hot rolling-cold rolling step as mentioned below is required in order to
ensure good properties. That is, it is necessary that the slab is
subjected to a finish rolling at 700.degree..about.900.degree. C. and
further to a coiling in a temperature range of
300.degree..about.600.degree. C. in the hot rolling of the steel slab.
The lower limit of the finish temperature is determined from a viewpoint of
suppressing the degradation of r-value due to residual strain, while the
upper limit thereof is determined from a viewpoint of preventing the
degradation of r-value due to the coarsening of crystal grain.
On the other hand, when the coiling temperature is too high, the improving
effect of the weldability with the coexistence of Nb and B becomes
considerably weak, so that the upper limit of the coiling temperature is
600.degree. C. When the coiling temperature is too low, troubles are
caused at subsequent steps, so that the lower limit is 300.degree. C.
The cold rolling is to impart an adequate cold strain required in the
formation of recrystallization texture. Therefore, the lower limit of the
reduction is 60% so as to provide a sufficient rolling strain. On the
other hand, when the reduction is too high, the loading of the rolling
machine becomes large and the productivity lowers, so that the upper limit
is 85%.
Moreover, the annealing temperature is required to be not lower than the
recrystallization temperature. However, when the annealing temperature is
too high, the steel is excessively softened and the effect aiming at the
invention can not be obtained, so that the upper limit is 780.degree. C.
On the other hand, when the fine Ti precipitates are existent in a great
amount, the recrystallization temperature and the softening temperature
shift toward high temperature side, so that the continuous annealing
temperature is shifted to 700.degree..about.900.degree. C. In the latter
case, the lower limit of 700.degree. C. is required to obtain a
recrystallization texture, while the upper limit of 900.degree. C. is
required to prevent the excessive softening of the steel sheet and the
coarsening of Ti precipitate.
According to the invention, when the fine Ti precipitates are dispersed
into the steel, it is not necessarily required to conduct the skin pass
rolling, but the skin pass rolling may be carried out at a usually
practised reduction. However, if it is intended to obtain a relatively
high Y.S. irrespective of Ti precipitate, the skin pass rolling becomes
particularly important. In FIG. 15 are shown results examined on the
influence of the reduction of skin pass rolling upon the lower limit of
reasonable welding current.
As a test steel, there were used various soft steel sheets for deep drawing
of 0.7 mm in gauge and the lower limit of weldable current thereof were
measured.
As seen from FIG. 15, the effect by the reduction of skin pass rolling is
particularly large in the Ti-Nb-B series steel, and there is recognized a
phenomenon that the lower limit of reasonable welding current is lower
than that of the low carbon steel when the reduction is not less than
(sheet gauge (mm)+0.1)%. Furthermore, the thus obtained steel sheet is
excellent in the fatigue properties of spot weld portion.
Then, there was examined the influence of the reduction of skin pass
rolling upon the fatigue properties at low cycle. As a test steel, there
were used four cold rolled and annealed steel sheets of 0.8 mm in gauge,
wherein the steel A was a general low carbon Al killed steel containing
0.04% of C, the steel B was an extra-low carbon steel containing 0.003% of
C and no Ti, Nb or the like, and the steel C was an extra-low carbon steel
according to the invention containing 0.002% of C, 0.031% of Ti, 0.007% of
Nb and 0.0006% of B, and the steel D was the same as in the steel C.
However, the steels A C were produced at a reduction of skin pass rolling
of 0.3%, while the steel D was produced at a reduction of skin pass
rolling of 1.3%.
The welding conditions were a welding time of 8 cycles, a welding current
of 7.5 kA and an applied pressure of 200 kgf. Furthermore, the addition
mode in the fatigue test was 0-tension or complete cantilevered shearing
tensile fatigue. The test was stopped according to JIS Z3136 when the
fatigue crack having a length equal to the nugget diameter was observed
from the steel sheet surface.
The measured results are shown in FIG. 16.
As seen from FIG. 16, the fatigue strength of the steel B as an extra low
carbon steel is low as compared with that of the steel A as a usual low
carbon steel. On the other hand, in the steel C containing Ti-Nb-B
subjected to skin pass rolling at a low reduction of 0.3%, the fatigue
strength at high cycle region is somewhat improved, but the fatigue
strength at low cycle region is still low. On the contrary, in the steel D
subjected to skin pass rolling at a high reduction of 1.5%, the fatigue
strength is largely improved at not only high cycle region but also low
cycle region.
Namely, it has been confirmed that such an effect is obtained when the
extra-low carbon steel contains Ti-Nb-B and subjected to skin pass rolling
at a high reduction likewise the case of FIG. 15.
According to the invention, it is necessary to conduct the skin pass
rolling at a reduction of not less than (sheet gauge (mm)+0.1)%. However,
when the reduction is too high, the degradation of mechanical properties
is conspicuous, so that the upper limit of the reduction is 3.0%.
Although the reason why the fatigue properties of spot weld portion are
effectively improved by subjecting to a skin pass rolling at the reduction
as mentioned above is not necessarily clear, it is guessed that the change
in the distribution of residual stress in thickness direction at the skin
pass rolling has some influence on the improvement of fatigue properties.
Moreover, it is desirable to control the surface of the steel sheet to the
aforementioned surface roughness by using a work roll having regulated
surface roughness in the skin pass rolling and/or cold rolling.
Although the laser dulling work has been mainly described as a dulling
process of the roll, plasma working, discharge working and the like may
naturally be utilized. In short, it is important that the surface
roughness should be included in the aforementioned reasonable range.
The function and effect of Ti, Nb and B addition on the weld portion are
summarized as follows.
At first, Ti is necessary for ensuring the mechanical properties to a
certain extent, fixing N and finely dispersing Ti precipitates. Secondary,
Nb makes up for the improving effect of mechanical properties through Ti,
and has an effect of forming fine structure together with B in addition to
the dispersing effect of Ti precipitates. Furthermore, B hardly has an
effect of forming the fine structure alone, but exhibits a remarkable
effect together with Nb or Ti precipitate.
Since the effect of forming the fine structure under the coexistence of Nb
and B is very strong, it is important that the amounts of Nb and B should
be restricted to a minimum while taking the balance among these elements.
Although the reason on the effect obtained by the combined addition of
Ti-Nb-B is not still clear, it is considered as follows.
In the spot welding, the steel sheet is locally fused and the temperature
in the vicinity of the fused portion becomes fairly high. In the extra low
carbon steel sheet, therefore, the crystal grains are generally and
considerably coarsened. This is a cause that the structure of the
conventional extra-low carbon steel is unsound, and a greatest cause that
the strength of the weld portion is low.
However, it has been confirmed that the structure in the vicinity of the
weld portion is not coarsened but is made fine in the steels according to
the invention. This is guessed due to the fact that a pair of Nb and B
atoms strongly suppresses the formation and growth of transformation
nucleus at .delta.-.gamma. or .gamma.-.alpha. transformation. In this
case, the structure of the weld portion is not a regular system but is a
needle system, which is a very rare structure as the extra-low carbon
steel.
The greatest feature of the invention lies in a point that the above effect
of forming the fine structure is obtained without causing the degradation
of the mechanical properties.
Furthermore, the presence of fine Ti precipitate propels the occurrence of
crystal grain nucleus for the .gamma.-formation at the heating state of
the spot welding and suppresses the growth of the grains at subsequent
step. Even at the cooling, the coarsening of the .gamma.-grains is
suppressed by the fine Ti precipitate dispersed into steel, and also the
fine and dense structure of the weld portion is obtained by the Ti
precipitate and the combined addition of Nb and B in the transformation at
the cooling. Thus, the excellent low-temperature roughness of the weld
portion can be obtained while holding the strength at a level equal to
that of the base metal.
Moreover, steel sheets additionally added with Ti, Nb or B for the purpose
of improving the deep drawability, secondary work brittleness and the like
and methods thereof are proposed in Japanese Patent Application
Publication No. 60-47,328, Japanese Patent laid open Nos. 59-74,232,
59-190,332, 59-193,221, 61-133,323 and the like. All of these conventional
techniques are to provide a good deep drawability by utilizing the
function and effect of each of Ti, Nb and B, from which the improving
effect of the spot weldability most importantly aiming at the invention
and further the fatigue properties of the weld portion can not completely
be expected.
For example, in each of the above articles, B is added for improving only
the bake hardnenability and secondary workability, while Nb is added for
restraining the ageability at room temperature and Ti is mainly added for
improving the mechanical properties. Therefore, the addition effects of
these elements are simply additional as a principle. Moreover, the
restrict condition of Ti+Nb<0.04% disclosed in Japanese Patent laid open
Nos. 59-74,232 and 61-133,323 or Ti+Nb<0.06% disclosed in Japanese Patent
laid open Nos. 59-190,332 and 59-193,221 is to merely give the good
phosphatability to the steel sheet. Therefore, the technical idea of
Ti-Nb-B combined addition considering the interaction among these elements
for the achievement of improved spot weldability aiming at the invention
can not completely be found in the above conventional techniques. Of
course, these conventional techniques disclose only the steels having a
chemical composition different from the composition defined in the present
invention [B: 0.0001.about.0.0010%, Nb: 0.001.about.0.010%, Ti:
0.01.about.0.04%, and B: (11/93)Nb-0.0004.about.(11/93)Nb+0.0004%,
Ti/(48/12.multidot.C+48/14.multidot.N)>1, Nb<1/5.multidot.(93/48)Ti] and
do not utterly disclose the improvement of spot weldability by the control
of distribution state of Ti precipitate or the control of the surface
roughness as in the invention at all. This is more clear from the Examples
disclosed in these conventional techniques. And also, it is apparent that
the Ti amounts claimed in the above conventional techniques,
Ti<48/14.multidot.N and Ti<48/12.multidot.C+ 48/14.multidot.N do not
completely satisfy the requirement defined in the invention.
It is needless to say that the conditions in the production of the steel
sheet disclosed in the conventional techniques are entirely different from
those of the invention because the properties and chemical composition of
the steel sheet are different from those of the invention as mentioned
above. As to the coiling temperature, however, Japanese Patent laid open
No. 59-74,232 discloses that the coiling temperature of not lower than
650.degree. C. is necessary, while Japanese Patent Application Publication
No. 60-47,328 and Japanese Patent laid open Nos. 59-190,332, 59-193,221
and 61-133,323 propose the coiling temperature of higher than 600.degree.
C. It is well-known that when the coiling temperature is made higher as
mentioned above, the mechanical properties are improved to a certain
extent, but various problems such as degradations of descaling property
and surface properties and the like are caused. On the other hand, the
invention is to improve these problems when using such a high coiling
temperature disclosed in these conventional techniques.
The following examples are given in the illustration of the invention and
are not intended as limitations thereof.
EXAMPLE 1
A molten steel having a chemical composition shown in the following Table 3
was continuously cast to form a cast slab. The resulting slab was cooled
at a cooling rate of 0.5.degree..about.5.degree. C./min over a temperature
range of 1,300.degree..about.1,000.degree. C. to produce various slabs
having different grain sizes of Ti precipitate.
TABLE 3
__________________________________________________________________________
C Si Mn P S N Ti Nb B Al
__________________________________________________________________________
0.0025
0.01
0.13
0.010
0.009
0.0027
0.012
0.007
0.0009
0.021
__________________________________________________________________________
Then, each of the slabs was heated to 1,150.degree. C., which was subjected
to a hot rolling, a cold rolling and further a continuous annealing at a
temperature of 770.degree. C.
In all of the cold rolled sheets cooled at a cooling rate of not less than
3.0.degree. C./min over the temperature range of
1,300.degree..about.1,000.degree. C. at the cooling stage of the slab, the
Ti precipitates having a grain size of not more than 0.05 .mu.m were
dispersed into steel in an amount of not less than 30 ppm as a Ti
conversion amount.
After each of the above cold rolled sheets was subjected to a spot welding
under the same conditions, the change of brittle temperature was measured
by a hammering test using a chisel to obtain results as shown in FIG. 9 as
a relation to the Ti conversion amount in the Ti precipitates having the
grain size of not more than 0.05 .mu.m.
As seen from FIG. 9, when the fine Ti precipitates are dispersed into steel
in an amount of not less than 30 ppm as Ti conversion amount, the
hammering brittle temperature is very low.
Then, the fracture surface at the hammering test was observed by means of
SEM (scanning electron microscope) to obtain a result as shown in FIG. 10
as a relation between fracture unit and hammering brittle temperature.
As seen from FIG. 10, the improvement of low-temperature toughness in the
steel according to the invention is considered to be based on the fact
that the fracture unit is made small in the formation of the fine
structure.
EXAMPLE 2
A molten steel having a chemical composition shown in the following Table 4
was continuously cast to form a cast slab, which was cooled at various
cooling rates shown in Table 4 over a temperature range of
1,300.degree..about.1,000.degree. C. at the solidification and cooling
stage of the slab and then heated to a temperature shown in Table 4.
Thereafter, the thus treated slab was subjected to a hot rolling, a cold
rolling and further continuous annealing at a temperature of
750.degree..about.800.degree. C.
The amount of fine Ti precipitate having a grain size of not more than 0.05
.mu.m as Ti conversion amount and the mechanical properties in the
resulting cold rolled sheets were measured to obtain results as shown in
Table 4.
Furthermore, the lower limit of reasonable welding current as well as T.S.
and vTrs of weld portion when these steel sheets were subjected to a spot
welding or a TIG welding were measured to obtain results as shown in the
following Table 5.
TABLE 4
__________________________________________________________________________
C Si Mn P S Ti Nb B Al N Ti/N
Symbol (ppm)
(wt %)
(wt %)
(wt %)
(wt %)
(wt %)
(wt %)
(ppm)
(wt %)
(ppm)
(wt %/wt
__________________________________________________________________________
%)
A Acceptable
26 0.01
0.16
0.010
0.009
0.013
0.006
8 0.022
26 2.3
B Example
33 0.02
0.14
0.010
0.010
0.018
0.007
7 0.024
29 6.2
C 31 0.02
0.15
0.011
0.012
0.023
0.004
3 0.021
34 6.8
D 24 0.01
0.15
0.011
0.008
0.031
0.006
9 0.022
39 7.4
E Comparative
22 0.02
0.16
0.011
0.007
0.008
0.007
8 0.024
24 3.3
F Example
32 0.01
0.16
0.011
0.007
0.052
0.007
8 0.025
38 13.7
G 31 0.01
0.16
0.010
0.009
0.021
0.021
9 0.028
31 6.7
H 20 0.01
0.15
0.009
0.011
0.024
0.006
18 0.025
36 6.7
I 23 0.01
0.15
0.009
0.009
0.016
-- 6 0.022
27 5.9
J 27 0.02
0.16
0.010
0.012
0.026
0.008
-- 0.023
25 10.4
K 29 0.02
0.16
0.009
0.008
-- 0.007
5 0.024
27 0
L 230 0.01
0.15
0.009
0.008
0.033
0.007
9 0.022
46 7.2
M Acceptable
30 0.02
0.16
0.009
0.009
0.029
0.007
6 0.025
32 9.1
N Example
36 0.03
0.15
0.010
0.010
0.035
0.006
9 0.026
28 12.5
O 25 0.01
0.14
0.011
0.008
0.022
0.009
9 0.025
24 9.2
P Comparative
29 0.02
0.15
0.010
0.009
0.023
0.006
9 0.027
38 6.1
Example
__________________________________________________________________________
Amount of Ti precipi-
Cooling
Heating
tate (.ltoreq.0.05 .mu.m) (Ti con-
rate temperature
Y.S. T.S. El
Symbol version amount wt %)
(.degree.C./min)
(.degree.C.)
(kgf/mm.sup.2)
(kgf/mm.sup.2)
(%)
r
__________________________________________________________________________
A Acceptable
0.009 3.5 770 19.3 30.2 51 2.2
B Example
0.012 3.3 780 21.2 30.3 50 2.1
C 0.018 3.3 775 19.1 31.2 48 2.3
D 0.021 3.6 770 19.6 32.8 50 2.2
E Comparative
0.002 3.1 780 15.3 29.1 50 2.1
F Example
0.005 3.7 785 15.5 31.9 47 1.9
G 0.014 4.1 775 20.5 30.8 49 2.0
H 0.015 3.4 780 19.8 31.2 48 1.8
I 0.009 3.6 785 19.5 30.3 46 1.1
J 0.013 3.7 785 18.9 29.4 45 2.1
K 0 3.5 770 14.6 28.3 44 2.0
L 0.021 4.2 775 19.5 34.5 32 1.2
M Acceptable
0.006 3.5 780 18.4 31.2 49 2.1
N Example
0.007 3.9 785 18.9 30.8 50 2.0
O 0.006 4.4 770 17.9 31.2 48 2.0
P Comparative
0.0025 1.5 730 15.2 29.4 49 1.9
Example
__________________________________________________________________________
TABLE 5
______________________________________
Spot welding TIG welding
Lower limit Charpy impact
of reason- strength of
able welding
Welding 2 mmV notch
current* strength**
vTrs (.degree.C.)
Symbol (kA) (kgf) WM Bond
______________________________________
A Accept- 6.0 400 -20 -30
B able 5.5 425 -25 -30
C Example 6.0 400 -25 -35
D 5.5 410 -30 -40
E Compar- 7.0 220 0 0
F ative 7.0 230 +10 +5
G Example 5.0 270 +5 0
H 5.5 270 0 +5
I 5.5 280 -5 0
J 6.0 200 0 0
K 7.5 250 +10 +5
L 6.0 390 0 -5
M Accept- 6.0 395 -15 -20
N able 6.0 390 -20 -25
O Example 6.5 400 -25 -30
P Compar- 7.0 210 +5 +10
ative
Example
______________________________________
*8 cycles 200 kg
**shearing tensile strength at 8 cycles, 200 kg and 8 kA
As seen from Tables 4 and 5, the mechanical properties as well as the
properties of weld portion and the weldability are poor in Comparative
Examples E.about.L having a chemical composition outside the reasonable
range defined in the invention. Furthermore, when the chemical composition
is within the reasonable range but the cooling rate is lower than the
lower limit defined in the invention (Comparative Example P), the good
properties are not obtained.
On the other hand, when the chemical composition and the treating
conditions are within the reasonable ranges defined in the invention
(Acceptable Examples A.about.D and M.about.O), not only the mechanical
properties but also the properties of weld portion and the weldability are
improved.
EXAMPLE 3
A continuously cast slab having a chemical composition shown in the
following Table 6 was heated to 1,250.degree. C. and subjected to a finish
hot rolling at 880.degree. C. to form a hot rolled sheet of 3.2 mm in
thickness, which was coiled at 550.degree. C. Then, the coiled sheet was
subjected to a cold rolling at a reduction of 75% to form a cold rolled
sheet of 0.8 mm in gauge, which was subjected to a continuous annealing at
a temperature of 750.degree. C.
The mechanical properties of the thus obtained steel sheets as well as the
lower limit of reasonable welding current and the welding strength were
measured to obtain results as shown in the following Table 7.
Moreover, each of the mechanical properties was represented by an average
value in the rolling direction, a direction of 45.degree. with respect to
the rolling direction and a direction perpendicular to the rolling
direction at a ratio of 1:2:1. The spot welding was carried out by using a
CF type electrode of 4.8 mm in diameter at a welding time of 8 cycles and
an applied pressure of 200 kgf. The welding strength was evaluated by a
value at a welding current of 7.5 kA.
TABLE 6
__________________________________________________________________________
Kind of steel C Si Mn P S Al N Ti Nb B
__________________________________________________________________________
A Acceptable
0.0024
0.01
0.15
0.010
0.010
0.018
0.0022
0.023
0.006
0.0007
Example
B Acceptable
0.0030
0.01
0.14
0.010
0.012
0.021
0.0028
0.035
0.008
0.0008
Example
C Acceptable
0.0015
0.01
0.14
0.011
0.007
0.020
0.0020
0.029
0.004
0.0006
Example
D Acceptable
0.0019
0.01
0.14
0.010
0.010
0.024
0.0023
0.017
0.006
0.0009
Example
E Acceptable
0.0028
0.01
0.13
0.010
0.008
0.021
0.0031
0.023
0.004
0.0003
Example
F Comparative
0.0017
0.01
0.15
0.011
0.008
0.025
0.0021
-- 0.005
0.0005
Example
G Comparative
0.0022
0.01
0.13
0.010
0.007
0.020
0.0035
0.033
-- 0.0003
Example
H Comparative
0.0038
0.02
0.15
0.011
0.011
0.021
0.0030
0.030
0.007
--
Example
I Comparative
0.0020
0.01
0.15
0.011
0.010
0.019
0.0025
0.072
0.005
0.0007
Example
J Comparative
0.0030
0.01
0.10
0.010
0.010
0.019
0.0019
0.005
0.006
0.0006
Example
K Comparative
0.0021
0.01
0.15
0.010
0.008
0.022
0.0020
0.031
0.052
0.0007
Example
L Comparative
0.0024
0.01
0.10
0.011
0.009
0.030
0.0022
0.025
0.005
0.0025
Example
M Comparative
0.0012
0.02
0.12
0.009
0.010
0.030
0.0015
0.020
0.004
0.0005
Example
__________________________________________________________________________
of steelKind
##STR1##
##STR2##
##STR3##
##STR4##
##STR5##
__________________________________________________________________________
A Acceptable
1.3 0.7 -0.00001
0.13 0.0050
Example
B Acceptable
1.6 0.9 -0.00015
0.12 0.0063
Example
C Acceptable
2.3 1.2 0.00013 0.07 0.0039
Example
D Acceptable
1.1 0.6 0.00019 0.18 0.0049
Example
E Acceptable
1.1 0.7 -0.00017
0.09 0.0058
Example
F Compara-
-- -- -0.00009
-- 0.0040
tive
Example
G Compara-
1.1 0.8 -- -- 0.0055
tive
Example
H Compara-
1.2 0.7 -- 0.12 (0.0064)
tive
Example
I Compara-
4.3 2.3 0.00011 0.04 0.0049
tive
Example
J Compara-
0.3 0.1 -0.00011
0.62 0.0053
tive
Example
K Compara-
2.0 1.1 -0.0055 0.87 0.0051
tive
Example
L Compara-
1.5 0.8 0.0019 0.10 0.0070
tive
Example
M Compara-
2.0 0.8 0.00033 0.10 0.0030
tive
Example
__________________________________________________________________________
TABLE 7
__________________________________________________________________________
No.
##STR6##
##STR7##
##STR8##
.sup.-r
current (kA)weldingreasonableLower limit
(kgf)strengthWelding
__________________________________________________________________________
A Acceptable
20.0 30.2 48.7
2.0
5.5 400
Example
B Acceptable
20.6 31.1 48.2
1.9
5.4 425
Example
C Acceptable
19.5 29.9 51.0
2.1
5.8 350
Example
D Acceptable
24.4 32.0 48.1
2.0
4.7 400
Example
E Acceptable
21.2 30.3 48.5
2.2
5.5 375
Example
F Comparative
17.2 32.2 46.5
1.8
6.6 300
Example
G Comparative
13.2 28.0 48.2
1.8
7.2 250
Example
H Comparative
11.0 28.8 48.8
1.9
6.1 200
Example
I Comparative
15.5 30.4 47.0
1.9
6.5 225
Example
J Comparative
14.5 30.1 48.0
1.6
6.7 300
Example
K Comparative
16.0 31.0 45.0
1.6
6.1 300
Example
L Comparative
16.0 31.6 16.1
1.6
6.9 275
Example
M Comparative
15.0 30.2 50.9
2.0
5.9 200
Example
__________________________________________________________________________
As seen from Table 7, all of Ti-Nb-B added extra-low carbon steel sheets
according to the invention (kind of steel: A.about.E) are not only good in
the r-value and El and excellent in the deep drawability, but also has a
wide lower limit of reasonable welding current in the spot welding and is
sufficient in the spot welding strength.
On the contrary, the spot weldability is poor in all of Comparative
Examples being outside the reasonable ranges defined in the invention.
EXAMPLE 4
A slab of steel having the same chemical composition as in the steel A of
Example 3 was treated under various conditions shown in the following
Table 8 to obtain cold rolled sheets (gauge: 0.8 mm).
The mechanical properties of the thus obtained sheets and the spot
weldability thereof were measured to obtain results as shown in the
following Table 9.
TABLE 8
__________________________________________________________________________
Finish
Coiling
Reduction
Continuous
Reduction
Temper-
Temper-
of cold
annealing
of skin
ature
ature
rolling
temperature
pass rolling
No. (.degree.C.)
(.degree.C.)
(%) (.degree.C.)
(%)
__________________________________________________________________________
1 Accept-
880 500 75 750 1.2
able
Example
2 Accept-
850 550 80 780 2.5
able
Example
3 Accept-
800 400 75 750 0.9
able
Example
4 Compar-
850 700 70 750 --
ative
Example
5 Compar-
850 500 75 880 0.3
ative
Example
6 Compar-
1000 550 70 770 0.4
ative
Example
7 Accept-
850 550 75 750 0.4
able
Example
8 Accept-
800 450 80 750 --
able
Example
__________________________________________________________________________
TABLE 9
__________________________________________________________________________
No.
##STR9##
##STR10##
##STR11##
.sup.-r
current (kA)welding reasonableLower limit
(kgf)strengthWelding
10.sup.5 cycles (kgf)low cycle
weldingFatigue strength
__________________________________________________________________________
at
1 Acceptable
20.2 30.4 48.6
2.2
5.5 400 230
Example
2 Acceptable
21.1 30.5 48.4
2.1
5.2 425 245
Example
3 Acceptable
19.2 30.1 48.8
2.1
5.9 375 225
Example
4 Comparative
18.8 30.2 47.7
1.7
6.1 275 165
Example
5 Comparative
16.9 29.8 48.6
1.9
7.0 200 150
Example
6 Comparative
18.0 30.8 45.5
1.6
6.7 300 160
Example
7 Acceptable
20.5 30.4 48.8
2.1
5.7 400 205
Example
8 Acceptable
19.5 30.2 49.0
2.1
5.9 375 200
Example
__________________________________________________________________________
As seen from Table 9, the steel sheets according to the invention (No.
1.about.3 and 7.about.8) exhibit good deep drawability and spot
weldability, while when the production conditions are outside the
reasonable ranges defined in the invention (No. 4.about.6), the mechanical
properties and the spot weldability are poor.
Particularly, the steel sheets according to the invention subjected to a
skin pass rolling at a high reduction (No. 1.about.3) have a high fatigue
strength at low cycle welding and exhibit more improved spot weldability.
EXAMPLE 5
A continuously cast slab of steel having a chemical composition shown in
the following Table 10 was heated to and soaked at 1,250.degree. C., and
then subjected to a rough rolling and a finish rolling to form a hot
rolled sheet of 3.2 mm in thickness. After pickling, the sheet was cold
rolled to obtain a cold rolled sheet of 0.7 mm in gauge, which was
subjected to a continuous annealing (soaking temperature:
750.degree..about.850.degree. C.) and further to a skin pass rolling
(reduction: 0.8%).
The skin pass rolling was carried out by using a work roll dulled through
laser working (laser dulled roll).
The surface roughness of the steel sheet was measured in the rolling
direction thereof, from which an average surface roughness SRa was
determined.
The mechanical properties of these steel sheets were measured to obtain
results as shown in the following Table 11.
Furthermore, the spot welding was carried out under conditions that the
welding time was 7 cycles, the applied pressure was 160 kgf and the
current was 6.5 kA, during which the spot weldability was evaluated by a
shearing tensile strength. The measured results are also shown in Table
11.
TABLE 10
__________________________________________________________________________
Steel
C Si Mn P S Ti Nb B Al N Remarks
__________________________________________________________________________
A 0.01
0.02
0.15
0.020
0.008
0.018
0.003
0.0006
0.03
0.0020
Comparative
Example
B 0.002
0.02
0.14
0.014
0.010
-- -- 0.0003
0.02
0.0020
Comparative
Example
C 0.002
0.02
0.16
0.016
0.013
0.020
0.005
0.0004
0.04
0.0015
Acceptable
Example
D 0.0015
0.02
0.14
0.021
0.017
0.030
0.004
0.0007
0.06
0.0015
Acceptable
Example
__________________________________________________________________________
TABLE 11
__________________________________________________________________________
Mechanical properties
Surface Shearing
roughness tensile
SRa Y.S. T.S. El strength
Steel
(.mu.m)
(kgf/mm.sup.2)
(kgf/mm.sup.2)
(%)
.sup.- r
(kgf)
Remarks
__________________________________________________________________________
A 1.0 20 36 42 1.3
213 Comparative
Example
B 1.1 19 35 44 1.4
241 Comparative
Example
C 0.7 18 32 49 1.9
267 Comparative
Example
1.5 16 29 51 2.2
304 Acceptable
Example
D 0.7 17 30 50 2.0
282 Comparative
Example
1.2 17 30 50 2.1
319 Acceptable
Example
__________________________________________________________________________
As seen from Table 11, the cold rolled steel sheets according to the
invention exhibit excellent press formability and spot weldability as
compared with those of the comparative examples.
EXAMPLE 6
A slab of steel having the same chemical composition as in the steels C and
D of Example 5 was produced in the same manner as in Example 5 and
subjected to the following test.
That is, the area ratio of convex portions and the average area per one
convex portion at the center face of surface roughness in the resulting
cold rolled steel sheets were measured by means of a three-dimensional
surface roughness meter.
The surface roughness and mechanical properties of the cold rolled steel
sheet are shown in Table 12.
TABLE 12
__________________________________________________________________________
Mechanical properties
Surface Shearing
roughness tensile
SSr
SGr Y.S. T.S. El strength
Steel
(%)
(.mu.m.sup.2)
(kgf/mm.sup.2)
(kgf/mm.sup.2)
(%)
.sup.- r
(kgf)
Remarks
__________________________________________________________________________
C 65 3 .times. 10.sup.4
18 31 49 1.8
263 Comparative
Example
50 1 .times. 10.sup.4
17 30 50 1.9
278 Comparative
Example
45 8 .times. 10.sup.4
18 30 50 1.8
335 Acceptable
Example
D 70 8 .times. 10.sup.4
19 33 45 1.7
250 Comparative
Example
30 7 .times. 10.sup.4
17 31 51 2.0
327 Acceptable
Example
__________________________________________________________________________
As seen from Table 12, the all steel sheets according to the invention
exhibit excellent press formability and spot weldability as compared with
those of the comparative examples.
As mentioned above, according to the invention, the extra-low carbon steel
sheets having an improved spot weldability can be obtained without
damaging the formability, so that they are suitable for use in
applications subjected to spot welding after the press forming such as
steel sheets for automobiles and the like.
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